It is crucial to determine a degradation degree of a lubricating oil in order to prevent an in-use machine from being broken and to judge a timing of changing the lubricating oil. For this judgment, the degradation degree of the lubricating oil is determined according to various methods. Examples of the methods include a method of measuring a kinematic viscosity change ratio, acid value increasing amount, foaming amount, water amount, anticorrosive performance, ASTM color, Rotary Pressure Vessel Oxidation Test (RPVOT) residual ratio, and the like described in ASTM D4378. In addition to the above method, the examples of the methods of determining the degradation degree further include, as an easy evaluation method: a method of measuring an antioxidant residual ratio by a voltammetry method described in ASTM D6810 and D6971; and a method of measuring a color difference (ΔE) by a color difference method described in ASTM D7843.
Further, in a turbine oil and wind-power generation, it is crucial to manage conditions of the lubricating oil in order to stably generate power. This is because an abrupt halt of power generation equipment caused by an oil solution or mixed-in foreign substances other than the oil solution significantly affects an economic activity and a civic lifetime. Moreover, in addition to managing conditions of the lubricating oil, a method of predicting a lifetime expectancy of the lubricating oil is also required.
However, in the method described in ASTM D4378, various measurements are required, a condition of the lubricating oil changes between at a sampling and at a measurement, and the measurements require skillfulness. Moreover, since the residual amount of the antioxidant contained in the lubricating oil is not determined in the method described in ASTM D4378, a substantial oxidation stability of the lubricating oil is unclear. Accordingly, the amount of the antioxidant needs to be separately determined and is generally measured by high speed liquid chromatography (HPLC). As a method using HPLC, for instance, there has been proposed a method including determining the amount of the antioxidant using HPLC and determining the amount of the antioxidant by gel permeation chromatography-ultraviolet photometric detector method (see Patent Literature 1).
Moreover, it has been proposed to subject the used lubricating oil to multivariable analysis using a near-infrared spectrum to predict properties and performance (see Patent Literature 2).